Method for treating recycled polyethylene terephthalate containing melting contaminants

ABSTRACT

A process for treating RPET flakes comprises providing a quantity of RPET flakes having melting contaminants, comminuting the RPET flakes to prepare RPET particles having an average mean particle size less than about 300 microns, melting the RPET particles to prepare an RPET melt, and mixing the RPET melt.

CROSS REFERENCE TO RELATED APPLICATION

This Application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/459,655 filed on Apr. 2, 2003.

FIELD OF THE INVENTION

This invention relates generally to a process for treating recycledpolyethylene terephthalate (RPET) containing contaminants. Moreparticularly, the invention is directed to a process for treatingcontaminated RPET, so that the small amounts of melting contaminantscontained in the RPET flakes have a negligible effect on the articleultimately produced from the RPET melt.

BACKGROUND OF THE INVENTION

Post-consumer processing of recycled PET to manufacture a variety oflow-tech consumer products such as flower pots and fence posts iswell-known. Typically, the recycling process utilizes used PETcontainers, such as discarded carbonated beverage containers, which arecollected, sorted, washed, and separated from contaminants to yield arelatively clean source of RPET. Additionally, the manufacture ofimperfect and damaged molded PET products, particularly the blow moldedbottles used for containing consumer goods, results in a considerableamount of PET waste which the manufacturers of such products would liketo reuse. The RPET produced by conventional recycling processes isgenerally in ground or flake form, which is thereafter melt processed orfurther pelletized by the end user.

RPET is generally subjected to a grinding operation in order to make thematerial easier to handle and process. Conventional grinding equipmentreduces the RPET to about ⅜ inch particles or flakes. The grinding isconducted in a manner to insure that a consistent flake size will beproduced, by employing a grate or screen through which the groundmaterial must pass upon exiting the grinder. Although conventional RPETflakes melt processing and pelletizing equipment is designed to handle ⅜inch flakes, some RPET materials having sizes as large as ½ inch and assmall as ¼ inch are also commercially produced. The bulk density of ⅜inch flake RPET generally ranges from about 22 to about 35 pounds percubic foot.

Due to the nature of the recycling process, numerous polymericcontaminants are typically found in RPET flakes. These contaminants maybe classified generally as either melting or non-melting contaminants.Melting contaminants are those which melt at or below the meltingtemperature of polyethylene terephthalate (PET), while non-meltingcontaminants are those which melt at temperatures above the melttemperature of PET. Examples of melting contaminants include PVC, Nylon,Polyethylene, Polypropylene, EVOH, Polystyrene, and the like.

It is known that melting contaminants degrade rapidly in the RPET meltstream of, for example, a melt extruder. Melting contaminants oftencause yellowing and a loss of intrinsic viscosity in the RPET plasticstream. Likewise, such contaminants often form discrete areas of verydark inclusions or gels within the RPET plastic matrix.

It would be desirable to treat contaminated RPET, so that small amountsof melting contaminants would have only a negligible effect on RPETmelting operations.

SUMMARY OF THE INVENTION

Accordant with the present invention, a process for treating RPET flakeswhich contain melting contaminants, so that the melting contaminantshave only a negligible effect during the RPET melting and formingoperations, has surprisingly been discovered. The process comprisesproviding a quantity of RPET flakes having melting contaminants,comminuting the RPET flakes to prepare RPET particles having an averagemean particle size less than about 300 microns, melting the RPETparticles to prepare an RPET melt, and mixing the RPET melt. By theterminology “RPET flakes having melt contaminants” as used hereinaboveis meant either RPET flakes mixed together with melting contaminants orRPET flakes containing imbedded contaminants. Accordingly, by theterminology “comminuting the RPET flakes” as used hereinabove is meantsimultaneously comminuting the RPET flakes and the mixed or imbeddedmelting contaminants.

The inventive process is particularly useful for preparing a polymermelt from RPET flakes, which melt is ultimately used for the processingand forming of, for example, plastic containers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is directed to a process for treating RPET flakes,comprising providing a quantity of RPET flakes having meltingcontaminants, comminuting the RPET flakes to prepare RPET particleshaving an average mean particle size less than about 300 microns,melting the RPET particles to prepare an RPET melt, and mixing the RPETmelt.

By the term “RPET flakes” as it is used herein is meant generally thecommercially available recycled polyethylene terephthalate materialsproduced by conventional PET recycling methods, usually in flake form,but which may additionally be in the form of chunks, spheres, pellets,and the like, and which are generally made available in bulk in asubstantially uniform particle size from about ¼ inch to about ½ inch.

According to the present invention, a quantity of RPET flakes containingmelting contaminants is provided for further processing. The quantity ofRPET flakes provided in the initial step of the inventive process mayeasily be determined by a routineer in the art of polymer processing,depending upon the quantity of polymer melt ultimately desired.

According to the present invention, the RPET flakes are comminuted byany conventional means to prepare RPET particles having an average meanparticle size less than about 300 microns. Methods and apparatus forcomminuting RPET and other polymer flakes are well known.

Following combination of the RPET flakes, the resultant RPET particlesare melted by conventional means such as, for example, by adding theneet or pelletized RPET particles to a melt extruder, or to a high sheardevice. As will be readily apparent to one ordinarily skilled in theart, the melting contaminants are melted along with the RPET particles.

Finally, the RPET melt is thoroughly mixed together by conventionalmeans. Conveniently, if a melt extruder or high shear device is used inthe previous step, the mixing will occur substantially simultaneouslywith the mixing operation.

This polymer blend component is beneficial for combining with otherpolymer materials for subsequent melt processing and forming operations.For example, the addition of the inventive RPET melt to a quantity ofvirgin polyethylene terephthalate will extend the volume of the virginPET.

If an RPET melt were prepared directly from the original RPET flakes,the resulting melt would be considered unusable or of low quality. Theinventive process, however, converts this erstwhile useless RPETmaterial into a higher grade of RPET which may even be useful for themanufacture of food grade containers.

The process for treating RPET flakes containing melting contaminantsdescribed hereinabove is generally disclosed in terms of its broadestapplication to the practice of the present invention. Occasionally, theprocess conditions as described may not be precisely applicable to eachRPET/contaminant combination included within the disclosed scope. Thoseinstances where this occurs, however, will be readily recognized bythose ordinarily skilled in the art. In all such cases, the process maybe successfully performed by conventional modifications to the disclosedmethod.

The invention is more easily comprehended by reference to specificembodiments recited hereinabove which are representative of theinvention. It must be understood, however, that the specific embodimentsare provided only for the purpose of illustration, and that theinvention may be practiced otherwise than as specifically illustratedwithout departing from its spirit and scope.

1. A process for treating RPET flakes, comprising: providing a quantityof RPET flakes having melting contaminants; comminuting the RPET flakesto prepare RPET particles having an average mean particle size less thanabout 300 microns; melting the RPET particles, to prepare an RPET melt;and mixing the RPET melt.
 2. The process for treating RPET flakesaccording to claim 1, wherein the RPET flakes comprise chunks, spheres,pellets, or mixtures thereof.
 3. The process for treating RPET flakesaccording to claim 1, wherein the RPET flakes have particle sizes fromabout ¼ inch to about ½ inch.
 4. The process for treating RPET flakesaccording to claim 1, wherein the melting contaminants comprisePolyvinyl Chloride, Nylon, Polyethylene, Polypropylene, Ethyl VinylAlcohol, Polystyrene, or mixtures thereof.
 5. The process for treatingRPET flakes according to claim 1, wherein the RPET particles are meltedin an extruder.
 6. The process for treating RPET flakes according toclaim 1, wherein the RPET particles are melted and the RPET melt ismixed simultaneously.